The present invention relates to methods of forming semiconductor devices, and more particularly, to methods of forming device isolation layers using STI (shallow trench isolation).
Conventionally, in order to electrically separate devices in a semiconductor chip, an STI (shallow trench isolation) process in which trenches are formed in a semiconductor substrate and buried with an insulating layer is widely used. FIG. 1 is a cross-sectional view illustrating a conventional STI device isolation layer. Referring to FIG. 1, a sidewall oxide layer 22 and a liner nitride film 24 are formed on sidewalls of a trench 15 of a semiconductor substrate 10 in a cell region, an NMOS region, and a PMOS region, and the trench 15 is buried by a device isolation insulation layer 30 on the liner nitride film 24.
The liner nitride film 24 may improve the refresh characteristics of a cell by reducing stress of the semiconductor device and preventing diffusion of doping elements such as boron. A silicon oxide layer used as the device isolation insulating layer 30 has a tensile stress, and thus a channel of an active region surrounded by the device isolation insulating layer 30 receives a compression stress. In particular, when an NMOS device receives a compression stress, electron mobility may be decreased, which may cause a decrease in the speed of the device. The liner nitride film 24 may reduce stress applied by the device isolation isolating layer 30 to the NMOS device.
However, in the case of a PMOS device, as illustrated in FIG. 2, after a transistor 40 is turned on, EHP (electron hole pairs), which are hot carriers, are created, and electrons e− of the EHP may be trapped in the liner nitride film 24. Accordingly, HEIP (hot electron induced punch through), i.e., a punch through caused by the electrons e− trapped in the liner nitride film 24, may occur. Holes h+ may be induced at a channel edge of the active region by the electrons e− trapped in the liner nitride film 24, which may cause deterioration of off characteristics of the device, for example, may increase currents when the transistor 40 is turned off.